This invention relates to the field of currency production, and in particular to a composite metal laminate coin planchet for use in the production of silver-colored coins.
A coin planchet, the flat disk of metal from which a coin is made, must satisfy several criteria to be useful and commercially desirable. First, the planchet must be the proper color. It has become established in many countries that low value coins have a copper color, medium value coins have a silver color, and high value coins have a gold color. Therefore, the color of the materials used in the production of coin planchets is vitally important to the usefulness of the planchet.
Next, because color is so important to the circulation of the coinage, a coin planchet must have a finish that does not significantly tarnish or wear over years of time in circulation. If excessive tarnishing occurs, the coin may become difficult to identify and, therefore, undesirable to the public. Also, if the planchet is not wear resistant, the raised portions of a coin created from the planchet may wear off prematurely, greatly diminishing the circulation life of the coin.
Another important criteria is the weight of the coin planchet. For example, many modem vending machines differentiate coinage by their weight. Therefore, if a dime planchet has the weight of a quarter, a vending machine will not properly distinguish the coin as a dime. Further, if a coin is lighter than approximately 2 grams, the coin will generally not trigger the weighing mechanism inside a vending machine. If multiple light coins are inserted into the machine, the coins will collect on the mechanism, eventually causing the machine to jam and require servicing. The weight of the coin planchet is also important to ensure circulation of the coin by consumers, because extremely light or extremely heavy coins are difficult to handle and unsatisfactory to the public.
Additionally, the cost of the metal in a coin planchet should be low relative to the face value of the coin. The less expensive the metal in a coin, the greater seigniorage is gained by the minting process. Also, if the value of the metal in a coin becomes greater than the face value of the coin, the government is forced to change the size or makeup of the coin to lower the value of the metal in the coin so that the public will not sell the coins for the value of the metal inside them. Of course, changing the size or makeup of a coin is undesirable because it is time consuming and expensive.
The metal in a coin planchet must also be of sufficient hardness. A planchet must be hard enough to ensure the coin has a useful circulation life. However, it must be soft enough to be readily mintable, i.e., it must be soft enough to be readily deformed by coin dies during the minting procedure to impart the required insignia to the coin faces. If the metal is too hard, the expensive coin dies used to impart the insignia wear out too quickly, or an undesirable shallow impression is produced on the struck coin. To avoid shallow impressions, a hard coin planchet is typically heated to soften it before it is minted. This process is known as secondary annealing. If a planchet is hard enough to ensure a long circulation life but soft enough to not require secondary annealing, the seigniorage of the coin made from the planchet would be increased by the cost of the energy typically used to heat hard planchets.
Further, in the creation of composite coins, i.e., those with a core of one material and an outer cladding of another, it is desirable that the scrap produced by the planchet-making process may be used for some economically beneficial purpose. Generally, coin planchets are stamped from a strip or sheet of metal. Because coins are generally circular, this process leaves an unused web of material, which, if not useful for another purpose, is simply a waste product of the planchet-making process. However, if the unused material could be used to create some other product, the overall waste of the planchet-making process would be reduced. The overall cost of the process would then be decreased because less waste would have to be removed and the web byproduct could be sold. By decreasing the cost of the process, the seigniorage of the coin is increased.
United States silver-colored coins are currently made of two compositions. Nickel coins are created from an alloy containing 75% copper and 25% nickel. Dime, quarter, half-dollar, and dollar coins are composites, created by cladding a core of copper with the same alloy used in nickel coins. These compositions meet some of the desired attributes of coin planchets mentioned above, but the metal scrap that remains after planchets are stamped from these compositions does not have any economically beneficial use. Also, these compositions require secondary annealing. Furthermore, these compositions are relatively expensive, and a planchet composition that is less expensive is desirable.
Composite metal laminates, like that currently used for dime, quarter, half-dollar, and dollar coins, have found wide use in coinage. Composite metal coins having a dissimilar core and cladding are highly desirable commercially due to the fact that the beneficial characteristics of the core and cladding may be obtained in one coin. In an article made of a single material or alloy, modifying a property of the article without effecting undesired changes in other properties of the article is difficult. For example, if the magnetic properties of an alloy are changed by altering the composition of the alloy, the color or weight of the alloy may also be affected. However, by forming composites, the aesthetic properties of the cladding can be achieved while the bulk properties of the core material are retained. Therefore, by forming composite articles, one can more easily and cost-effectively change various properties of the article by modifying the composition or altering the thickness of the core or cladding.
The composite currently used for United States dime, quarter, half-dollar, and dollar coins fails, over an extended period of time, to satisfy the requirement of a consistent silver color. The extended wear that occurs over the life of a coin sometimes causes parts of the cladding to wear off, revealing the copper core beneath. Because the copper core is not silver in color, the coin must then be replaced. If a composite planchet was available that had both a core and a cladding of silver color, the many benefits of composite metal laminates could be realized and the circulation life of the coin made from the planchet would be increased.
Therefore, there is a need in the industry for a coin planchet which is uniformly silver in color and is composed of relatively inexpensive material. Further, the modification of a coin planchet property needs to be easy, cost-effective, and free from effecting undesired changes in other planchet properties. Additionally, there is a need for a coin planchet of sufficient weight with a finish that is tarnish and wear resistant. Also, a coin planchet is needed which is sufficiently hard, does not require secondary annealing, and is made of a composition such that scrap material from the planchet making process may be sold or used to make another product.
The present invention is directed to an article and a process that satisfy the need for an inexpensive, tarnish resistant, wear resistant, sufficiently hard, and sufficiently heavy composite metal laminate coin planchet which is silver in color, does not require secondary annealing, and has a composition such that the byproduct of its production process has economic value. The article comprises a planchet with two commercially pure aluminum cladding layers metallurgically bonded to a zinc alloy core. The zinc alloy core may contain about 0.5 percent to about 1.5 percent of a hardening agent, such as copper or titanium. This hardening agent increases the hardness of the alloy above that of pure zinc. Trace amounts of other elements may also be present in the zinc alloy core or in the commercially pure aluminum cladding layers which do not affect their pertinent properties.
The combination of the zinc alloy core and the commercially pure aluminum cladding layers is ideal for coin planchets, for the zinc alloy and commercially pure aluminum are inexpensive, silver in color, and sufficiently hard. The zinc alloy is heavier than the commercially pure aluminum, so the ratio of each in a coin planchet may be adjusted to achieve an acceptable weight. Further, the tarnish resistance of the commercially pure aluminum cladding layers compensates for the zinc alloy""s poor tarnish resistance. Also, a coin planchet comprised of such a zinc alloy core and commercially pure aluminum cladding layers is extremely wear resistant and does not require secondary annealing prior to minting.
The process used to create the coin planchet comprises providing a strip or sheet of zinc alloy core and then creating a strip or sheet of composite material by metallurgically bonding two commercially pure aluminum cladding layers to the strip or sheet of zinc alloy core. After bonding the cladding layers to the zinc alloy core, objects are removed from the composite material, such as coin planchets. The composite material that remains in the strip or sheet is then heated above its melting point. This heating process creates a zinc-aluminum alloy which may be sold to members of the zinc-aluminum die-cast industry. Therefore, the byproduct of the planchet production process earns revenue for the planchet producer, increasing the seigniorage of the planchet.
These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and claims.